1. Technical Field
The present invention relates to acoustics and more specifically to sound barrier structures.
2. Prior Art
Sound barrier walls typically used along highways have flat surfaces which simply reflect noise with limited change. Sound barriers with three dimensional surfaces have greater potential for controlling noise. Complex baffle and resonator structures for example are well-known in acoustic chamber applications. Several U.S. patents disclosing sound barriers are reviewed in Applicant's co-pending application Ser. No. 07/705,587, now abandoned, which is incorporated herein by reference.
Referring to FIG. 1, a conventional Helmholtz resonator R has a hollow cavity C of volume V mostly surrounded by inside surface S of enclosure E. Interior cavity C communicates by a proportionally dimensioned port P through an opening of diameter D and length L on axis T to the exterior of enclosure E.
Acoustic resonators R are conventionally molded around a given volume V defined by a convex outside surface of a rigid solid interior form (not shown).
Conventionally, molding media mass is supported around rigid forms. After molding a medium outside, any solid rigid forms left inside need to be removed to embody a resonator R. Interior form removal may be possible only in pieces through a port compromised to a larger than preferred diameter D.
Cavity volumes V can as well be defined on the exterior of a molding medium using rigid forms with concave inside surfaces. If external pressures can be neglected or controlled during molding, then a cavity C interior does not need support by a form.
For example, in blow molding, a fluid (e.g., air) pressure on an inside surface of a sheet of plastic overcomes a lower pressure on its outside surface of the plastic confronted by a rigid exterior form. Biased pressures on the sheet plastic distends outward to, and through a port hole entrance into, the form. A resonator cavity opening inflates inside to conform the plastic surface to bulb molding surfaces on the inside of the rigid form.
Obstacles such as these constrain the economy of using acoustic resonators more widely as noise barriers. Thus, there remains a need for improved techniques of constructing sound barriers.